Reinforced plastic pallet and method of making

ABSTRACT

A hollow upper deck panel for a plastic pallet has a solid top surface and is produced by forming a tubular hot plastic parison and directing the parison between blow mold sections to form upper and lower sheets for the deck panel. The lower sheet is blow molded by pressured air within the parison into a mold section to form spaced inverted U-shaped open ribs projecting upwardly and with top portions fused to the upper sheet. The upper sheet is blow molded by the air around retractable blades to form spaced closed double wall ribs projecting downwardly between the open ribs and have bottom portions fused to the lower sheet. The closed double wall ribs extend transversely to the open ribs and have end portions fused to the open ribs. The support structure for the upper deck panel defines spaces for receiving parallel spaced forks of a forklift truck.

BACKGROUND OF THE INVENTION

In the manufacture of large pallets of rigid plastics material and having recesses or spaces for receiving parallel spaced forks of a forklift truck, it is well known to form the pallets with a twin sheet thermoformed upper deck, for example, as disclosed in U.S. Pat. Nos. 5,197,396, 5,413,052, 5,676,064 and 5,813,355. It is also known to make or form upper decks and other components for a plastic pallet by extruding a plastic parison and directing the parison into blow molding equipment, for example, as disclosed in U.S. Pat. Nos. 5,845,588, 6,962,115 and 6,976,437. Blow-molding processes have also been used to form rigid hollow plastic panels, for example, as disclosed in U.S. Pat. No. 6,826,887. Plastic pallets have also been made by using injection molded components, for example, as disclosed in U.S. Pat. Nos. 6,389,990, 6,840,181 and 7,197,989. It is further known to make plastic pallets by rotational molding, for example, as disclosed in U.S. Pat. No. 6,807,911.

In the manufacture of any plastic pallet or hollow plastic panel, it is desirable to maximize the strength/weight ratio while minimizing the cost of manufacturing or producing the pallet or panel. Also, for many uses of a plastic pallet or panel, it is desirable for the top surface to be substantially smooth and solid without openings which can trap or collect undesirable debris or material. When the top deck and the bottom deck of a plastic pallet are vacuum-formed by a twin-sheet thermoforming process, it is also known to insert tubular metal reinforcing members between the sheets, for example, as disclosed in above mentioned U.S. Pat. Nos. 5,197,396 and 5,413,052. The top and bottom decks or panels of a plastic pallet formed by blow molding or by a twin-sheet or triple sheet thermoformed process are also commonly reinforced by forming integral hollow or open ribs in the top and/or bottom sheets, for example, as disclosed in above U.S. Pat. Nos. 5,676,064, 5,813,355 and 6,749,418.

SUMMARY OF THE INVENTION

The present invention is directed to a plastic pallet having a plastic top deck panel incorporating integral rib reinforcement which significantly increases the strength of the deck panel and which also helps to minimize the thicknesses of the plastic sheet material forming the panel. A top deck panel constructed in accordance with the invention also provides the pallet with a solid top surface without any recesses or cavities which can collect dirt and debris.

In accordance with the illustrated embodiment of the invention, a reinforced hollow upper deck panel for a plastic pallet is produced by a blow molding process wherein an extruded tubular parison of thermoplastic material may be directed downwardly from the extruder between opposing open mold sections of blow molding equipment so that the hot parison forms an upper sheet and a lower sheet for the upper pallet deck panel. After the mold sections close to fuse peripheral portions of the upper and lower sheets together, pressurized air within the parison forms the lower sheet into a first mold section to form parallel spaced inverted U-shaped elongated open ribs which project upwardly and have spaced side walls. The upper sheet of thermoplastic material is formed by the pressurized air into a second mold section which has a set of extendable and retractable parallel blades to form parallel spaced elongated closed double wall ribs projecting downwardly from the upper sheet and having adjacent side walls. After the mold sections close on the hot sheets formed by the tubular parison and the parison is pressurized with air and with optional vacuum assist in the mold sections, the top portions of the open ribs in the lower sheet fuse to the upper sheet, and the double wall closed ribs project downwardly between the open ribs and have bottom portions fused to the lower sheet.

While the air pressure continues between the fused together upper and lower sheets and before the sheets solidify, the blades are retracted so that the adjacent walls of the double walled ribs are fused together. The closed double walled ribs extend transversely or perpendicular to the open ribs, and the end portions of the double walled ribs fuse to the side walls of the open ribs, with the result that the hollow plastic upper deck panel is reinforced by the crossing ribs in all directions. The supporting structure for the upper deck panel is also preferably formed of plastics material, and a lower deck panel for the pallet may be formed in the same manner as the upper deck panel.

Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a plastic pallet having a reinforced hollow plastic panel constructed in accordance with the invention and forming the upper deck of the pallet;

FIG. 2 is a perspective section of the hollow plastic panel, taken generally on the line 2-2 of FIG. 1;

FIG. 3 is an enlarged perspective fragmentary section of a corner portion of the plastic panel shown in FIG. 2;

FIG. 4 is a fragmentary perspective section of the final blow molded upper deck panel shown in FIG. 1 and taken generally on the line 4-4 of FIG. 3;

FIG. 5 is a fragmentary horizontal section of blow molding tooling with the mold sections in an open position and a pressurized parison between the mold sections;

FIG. 6 is a larger fragmentary horizontal section of the tooling shown in FIG. 5;

FIG. 7 is a larger fragmentary horizontal section of the tooling shown in FIG. 5 with the mold sections in a closed position and a rib forming blade in an extended position; and

FIG. 8 is a fragmentary section similar to FIG. 7 and showing the rib forming blade in a retracted position.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a plastic pallet 10 having a lower deck panel 12 connected by a set of peripherally spaced hollow or tubular legs 14 to a hollow plastic panel 15 constructed in accordance with the invention and forming the upper deck for the pallet 10. As shown in FIGS. 5-8, the hollow upper deck panel 15 is formed by a blow molding method and tooling and equipment. The hollow panel 15 is formed from an extruded tubular or hollow parison 16 (FIG. 5) of heated thermoplastic material and which forms an upper thermoplastic sheet 18 (FIG. 5) and a lower thermoplastic sheet 20 for the upper deck panel 15. The hot parison 16 may be extruded downwardly directly into and between opposing mold sections of blow molding equipment or the parison may be made as a preform and later reheated before entering the blow mold sections. The heated lower sheet 20 is formed by pressurized air within the parison 16 into cavities 23 and 26 within a mold section 30 supported by parallel spaced beams 32. As shown in FIGS. 2 & 3, when the sheet 20 is formed into the cavities 23 and 26 and other cavities within the mold section 30, elongated inverted U-shape bottom open ribs 35 and 37 are formed within the sheet 20 along with a peripherally extending wall 42 (FIG. 6) extending to a peripherally extending and outwardly projecting lip or flange 44. Vacuum assist may also be used in the mold section 30 to aid in forming the open ribs 35 and 37.

The forming of the lower sheet also forms a series of peripherally spaced open bottom cavities 46 and 48 (FIGS. 2-4) within the lower sheet 20, and the cavities receive the upper end portions of the legs 14 when the lower deck panel 12 is later assembled to the upper deck panel 15. The lower deck panel 12 is also formed to have cavities corresponding to the cavities 46 & 48 to receive the lower end portions of the legs 14 which are later attached or fused to the lower deck panel 12 and upper deck panel 15 during final assembly of the pallet.

Referring to FIG. 5, the blow mold tooling also includes a mold section 55 which opposes the mold section 30 and includes a mold plate 57 rigidly connected to a mold plate 59 with the mold plate 57 supporting a fluid cylinder 62. The mold plate 57 and mold plate 59 are supported by parallel spaced beams 65 which are connected to a hydraulic cylinder (now shown) for movement as a unit for moving the mold sections between an open position (FIGS. 5 & 6) and a closed position (FIGS. 7 & 8). The mold plate 59 is also supported by a peripherally extending frame members 67 and parallel spaced members or rails 69 each having a passage 72 for receiving cooling fluid. A plate 74 is positioned between the plates 57 and 59 and has slots for receiving the support members or rails 69. The plate 74 is connected to the piston of the fluid cylinder 62 and is movable in response to actuation of the cylinder 62 between the plate 57 and plate 59. The plate 74 supports a set of parallel spaced thin blades 75 for movement between extended positions (FIGS. 5-7) projecting through corresponding slots within the plate 59 and retracted positions (FIG. 8) where the bottom ends of the blades 75 are flush with the bottom surface of the plate 59.

During the blow molding process, the air pressure in the parison 16 forms the heated sheet 18 into a cavity 78 within the plate 59 to provide the pallet 10 with a flat solid top surface 80 (FIGS. 1 & 4) surrounding by a peripherally extending vertical wall 82 (FIG. 6) which continues to form a peripherally extending and outwardly projecting horizontal lip or flange 84 for the panel 15. The blades 75 extend outwardly during the forming operation so that the heated sheet 18 forms a double wall rib 90 (FIGS. 5 & 6) around each of the blades 75. Vacuum assist may be used within the mold plate 59 to aid in forming the double wall ribs 90. After the sheet 18 and the sheet 20 have been formed and are still hot and not solidified, the mold section 55 shifts and closes on the mold section 30 (FIG. 7) when the peripheral connected flanges 44 & 84 fuse together, the upper end portions of the open ribs 35 & 37 fuse to the top sheet 18 of the pallet 10, and the lower end portions of the double wall ribs 90 fuse to the lower sheet 35. Also, when the mold sections close, the top walls of the cavities 46 & 48 fuse to the upper sheet 18.

The blades 75 are then retracted (FIG. 8), while the hollow space between the fused together sheets 18 and 20 and surrounding the parallel adjacent side walls of the ribs 90 continues to be pressurized with air so that the adjacent side walls of each rib 90 are fused together, as shown in FIG. 8, to form the closed and fused double wall ribs 90 shown in FIGS. 2, 3, 4 & 8. As also shown in FIGS. 2 & 3, the closed double wall ribs 90 extend transversely or perpendicular to the open ribs 35, and the opposite end portions of the closed double wall ribs 90 are fused to the adjacent side walls of the open ribs 35. After the upper deck panel 15 is formed by the blow molding process, and the panel 15 is removed from the open mold sections 30 & 55, the flat top surface 80 of the upper deck panel 18 is smooth and solid without any cavities. The surface 80 has only linear marks or lines 95 where the blades 75 have been retracted (FIG. 8) to form the closed and fused double wall ribs 90.

From the drawings and the above description, it is apparent that a hollow plastic upper deck panel for a plastic pallet and made in accordance with the invention, provides desirable features and advantages. As one advantage, the arrangement of the downwardly projecting closed double wall ribs 90 within the upper sheet 18 of the pallet deck panel 15 and the crossing open ribs projecting upwardly from the lower sheet 20 with the ribs fused to the opposing sheets, along with the fused peripheral flanges, provides the panel 15 with substantial strength in all directions while minimizing the weight of the panel. In addition, by forming the closed double wall ribs 90 within the sheet 18, the top surface of the panel is essentially solid, which is especially desirable for many uses of a plastic pallet. Also, by fusing the opposite ends of the double wall closed ribs 90 with the side walls of the vacuum-formed open ribs 35, the connected crossing connected ribs add further to the strength of the hollow deck panel 15. By increasing the strength of the hollow deck panel 15 in all directions, the thickness of the sheets 18 and 20 may be reduced depending on the load to be supported, with the result that the weight of the pallet may be reduced for transporting a predetermined load.

While the form of blow molded upper deck panel for a pallet herein described and its method of construction constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of deck panel, and that changes may be made therein without departing from the scope and spirit of the invention as defined in the appended claims. 

1. A plastic pallet comprising an upper deck panel of rigid plastics material and connected to a lower support structure of rigid plastics material with said support structure defining spaces adapted to receive parallel spaced forks of a forklift truck, said upper deck panel including an upper sheet of rigid plastics material and a lower sheet of rigid plastics material, said upper sheet and said lower sheet comprising opposite side portions of a tubular parison of extruded plastics material, said lower sheet of said upper deck panel including a series of parallel spaced inverted U-shape elongated open ribs projecting upwardly and having spaced side walls and top portions fused to said upper sheet to reinforce said upper deck panel in a first horizontal direction, said upper sheet of said upper deck panel including a series of elongated closed double wall ribs projecting downwardly between said parallel spaced open ribs in said lower sheet and having bottom portions fused to said lower sheet, and said closed double wall ribs in said upper sheet having fused together side walls and extending transversely to said open ribs in said lower sheet to reinforce said upper deck panel in a second direction transverse to said first direction.
 2. A plastic pallet as defined in claim 1 wherein said closed double wall ribs have opposite end portions intersecting and fused to said spaced side walls of said open ribs.
 3. A plastic pallet as defined in claim 1 wherein said closed double wall ribs extend perpendicular to said open ribs.
 4. A plastic pallet as defined in claim 1 wherein said upper sheet of said upper deck panel has a substantially solid top surface with parallel spaced linear marks corresponding to the locations of said closed double wall ribs.
 5. A plastic pallet as defined in claim 1 wherein said lower sheet includes upwardly projecting peripherally spaced portions having top walls fused to said upper sheet and side walls defining cavities for receiving leg members of said lower support structure.
 6. A plastic pallet comprising an upper deck panel of rigid plastics material and including an upper sheet of rigid plastics material and a lower sheet of rigid plastics material, said upper sheet and said lower sheet comprising opposite side portions of a tubular parison of extruded plastics material, said lower sheet of said upper deck panel including a series of parallel spaced inverted U-shape elongated ribs projecting upwardly and having side walls and top portions fused to said upper sheet to reinforce said upper deck panel in a first horizontal direction, said upper sheet of said upper deck panel including a series of elongated closed double wall ribs projecting downwardly between said parallel spaced ribs in said lower sheet and having bottom portions fused to said lower sheet, and said closed double wall ribs in said upper sheet having fused together side walls and extending transversely to said ribs in said lower sheet to reinforce said upper deck panel in a second direction transverse to said first direction.
 7. A plastic pallet as defined in claim 6 wherein said closed double wall ribs in said upper sheet have opposite end portions intersecting and fused to said side walls of said ribs in said lower sheet.
 8. A plastic pallet as defined in claim 6 wherein said closed double wall ribs extend perpendicular to said ribs in said lower sheet.
 9. A plastic pallet as defined in claim 1 wherein said upper sheet of said upper deck panel has a substantially solid top surface with parallel spaced linear marks corresponding to the locations of said closed double wall ribs.
 10. A method of producing a plastic pallet having a hollow plastic upper deck panel, comprising the steps of extruding a tubular parison of heated plastics material and directing the parison between open blow mold sections with the parison forming an upper sheet and a lower sheet for the upper deck panel, closing the mold sections and fusing corresponding peripheral edge portions of the upper sheet and the lower sheet together by heat and pressure, introducing a pressurized gas into the parison and forming a series of upwardly projecting parallel spaced inverted U-shape elongated ribs within the lower sheet with the ribs having side walls and top portions, fusing the top portions of the ribs in the lower sheet to the upper sheet to reinforce the upper deck panel in a first horizontal direction, extending a series of parallel spaced blades into the upper sheet and forming in the upper sheet a series of elongated double wall ribs projecting downwardly between the parallel spaced ribs in the lower sheet with each of the closed ribs having adjacent side walls and a bottom portion, retracting the blades and fusing the adjacent side walls of each of the double wall ribs together to form closed double wall ribs and fusing the bottom portions of the closed double wall ribs to the lower sheet, and positioning the closed double wall ribs in the upper sheet transversely to the ribs in the lower sheet to reinforce the upper deck panel in a second direction transverse to said first direction.
 11. A method as defined in claim 10 and including the step of fusing opposite end portions of the closed double wall ribs in the upper sheet to the side walls of the ribs in the lower sheet.
 12. A method as defined in claim 10 wherein the closed double wall ribs in the upper sheet are extended perpendicular to the ribs in the lower sheet.
 13. A method as defined in claim 10 and including the step of forming the upper sheet with a substantially solid top surface having parallel spaced linear marks corresponding to the locations of the closed double wall ribs.
 14. A method as defined in claim 10 and including the step of forming the lower sheet with upwardly projecting peripherally spaced portions having top walls fused to the upper sheet and side walls defining with the top walls downwardly facing open bottom cavities adapted to receive leg members of a lower support structure for the upper deck panel. 